The greenhouse effect due to CO2 has recently been pointed out as a cause of global warming, and international measures to address the greenhouse effect are urgently needed to protect the global environment. Sources of CO2 emissions are present in all areas of human activity in which fossil fuel is combusted, and the demand for reducing CO2 emissions tends to further increase. To meet the demand in power generation facilities such as thermal power plants that use a large amount of fossil fuel, intensive research has been conducted on a method of removing and recovering CO2 in flue gas from a boiler by bringing the flue gas into contact with an amine-based CO2 absorbent and on a method of storing recovered CO2 with no emissions to the air. One process used to remove and recover CO2 in flue gas using such a CO2 absorbent described above includes the steps of bringing the flue gas into contact with the CO2 absorbent in an absorber, heating the absorbent containing CO2 absorbed therein in a regenerator to release CO2 and regenerate the absorbent, and recirculating the regenerated absorbent into the absorber to reuse the absorbent (see, for example, Patent Literature 1).
With the method of absorbing, removing, and recovering CO2 in CO2-containing gas such as flue gas using the above-described CO2 absorbent and process, since the process is installed additionally in a combustion facility, it is necessary to reduce the operating cost of the process as much as possible. Particularly, the regeneration step in the above process consumes a large amount of thermal energy, and therefore the energy used in the process must be reduced as much as possible.
In one prior proposal, part of a semi-lean solution is drawn off from a regenerator to the outside. The drawn semi-lean solution exchanges heat with a lean solution in a heat exchanger and then exchanges heat with steam-condensed water in another heat exchanger. The resultant semi-lean solution is returned to a position downward of the drawn-off position. The temperature of the semi-lean solution supplied to the lower side of the regenerator is increased, and the amount of steam consumed is thereby reduced (see, for example, Patent Literature 2 (embodiment 8, FIG. 17)).
Regarding CO2 absorbents, in order to improve the performance thereof, absorbents contributing the improvement in their absorption performance have been proposed (Patent Literatures 3 and 4).